JP2908207B2 - Optical transmitter for wavelength multiplex transmission - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission apparatus constituting an optical communication system using an optical fiber amplifier, and more particularly to the application of wavelength multiplex transmission to an optical fiber amplifier using a plurality of optical signals having different center wavelengths. About.

[0002]

2. Description of the Related Art As a means for increasing the transmission capacity without increasing the number of transmission lines in an optical communication system, a wavelength division multiplex transmission system is widely known as a unique method of optical communication in addition to a time division multiplex transmission system. In this method, a plurality of optical signals having different center wavelengths are multiplexed by arranging an optical multiplexer on an optical transmitting side, transmitting the optical fiber transmission line, and then again using an optical demultiplexer on an optical receiving side. In this case, the transmission capacity is increased without increasing the number of optical fiber transmission lines.

On the other hand, in order to increase the transmission distance, a system for directly amplifying an optical signal using an optical fiber amplifier without converting the optical signal into an electric signal in a repeater is being put to practical use. In optical amplification using an optical fiber amplifier, not only is it applied to a method of amplifying a weak optical signal transmitted as used in a repeater, but also an optical transmission circuit and an optical fiber for transmitting an optical signal. There is a method in which this is arranged between transmission lines and used as a booster amplifier.

[0004] A system in which an optical fiber amplifier is arranged on the optical transmission side to increase the transmission distance and a system in which the transmission capacity is increased by wavelength division multiplexing transmission described above are being studied. As an example of research on such a method, for example, H.-K. Taga et. a
l. , "459km, 2.4Gb / s 4wavele
NgthMultiplexing Optical
Fiber Transmission Experi
ment using Er-doped Fiber
Amplifiers ", OFC '90 PD9-
1 1990.

FIG. 2 shows an example of a conventional optical transmission device for wavelength division multiplexing transmission, which is a configuration diagram of a device used for wavelength division multiplexing transmission of four waves. Optical transmission circuits 11a, 11b, 11
c and 11d have different center wavelengths λ ₁ , λ ₂ ,
The optical signals having λ ₃ and λ ₄ are respectively transmitted. here,
Each of the optical transmission circuits 11a, 11b, 11c, and 11d includes a light source that converts an electric signal into an optical signal, for example, a semiconductor laser, and an electric circuit that drives the light source. The transmitted optical signals having the respective center wavelengths are multiplexed by the optical multiplexer 15 and transmitted to the optical fiber 13, where the multiplexed light is optically amplified. The amplified multiplexed light is transmitted through the optical fiber transmission line 14 and input to the first optical demultiplexer. Then, it is demultiplexed into optical signals having the original center wavelengths λ ₁ , λ ₂ , λ 3, and λ ₄ , and the respective optical receiving circuits 12a, 12b, 12c, 12
d and converted into an electric signal. By using such a configuration, it is possible to wavelength-division multiplex and transmit an optical signal having a plurality of different center wavelengths to an optical communication system also using an optical fiber amplifier, thereby increasing the transmission distance. The transmission capacity is increased. In the above configuration, the optical fiber amplifier includes an amplification optical fiber, an excitation light source, an optical coupler, and an isolator (not shown), and an erbium-doped optical fiber is used as the amplification optical fiber.

[0006]

However, in general, when an optical signal having a plurality of center wavelengths is input to an optical fiber amplifier, gain competition occurs inside the optical fiber amplifier, and the optical fiber amplifier transmits the signal to the optical fiber transmission line. It is difficult to set the output of the transmitted optical signal of each center wavelength to a desired value. In particular, since the gain of an optical fiber amplifier depends on the wavelength, the gain is significantly different between a wavelength at which the gain is peak and a wavelength far from the wavelength. For this reason, the output of the optical signal transmitted to the optical fiber transmission line also has a large difference depending on the wavelength.

[0007] Such an output difference on the optical transmitting side appears as a difference in intensity input to the optical receiving circuit. Then, sufficient isolation characteristics between the respective optical signals after being split into optical signals of the respective center wavelengths by the optical splitter disposed on the receiving side cannot be obtained. That is, if there is a difference in the reception level of the optical signal between the respective center wavelengths on the optical receiving side, the intensity of the optical signal having the wavelength of the channel to be received and the wavelength of the adjacent channel after being demultiplexed into the respective wavelengths There is a case where the difference is not sufficient, and there is a problem that accurate reception is not possible due to the influence of the optical signal of the adjacent channel.

SUMMARY OF THE INVENTION In view of the above-mentioned disadvantages, an object of the present invention is to provide an optical communication system using an optical fiber amplifier, even if a wavelength division multiplexing transmission system is introduced, due to the wavelength dependence of the gain of the optical fiber amplifier. It is an object of the present invention to provide a wavelength division multiplexing transmission optical transmission device capable of suppressing degradation of isolation characteristics on the receiving side and performing high quality transmission.

[0009]

SUMMARY OF THE INVENTION In order to eliminate the above-mentioned drawbacks, an optical transmitter for wavelength division multiplex transmission according to the present invention converts a plurality of electric signals into a plurality of optical signals having different center wavelengths from each other. An optical transmission circuit, an optical multiplexer for multiplexing a plurality of optical signals and transmitting multiplexed light, an optical fiber amplifier for amplifying the multiplexed light and transmitting amplified light to an optical transmission line, and a part of the amplified light Optical splitter (hereinafter referred to as "optical splitter") for splitting the split light into optical signals of each wavelength, and an optical splitter for splitting the split light into optical signals of each wavelength. A plurality of O / E converters for converting the optical signals into electrical signals each time, and a plurality of electrical signals corresponding to the optical signals of each wavelength are compared to control the intensity of each optical signal transmitted from the optical transmission circuit. And a control circuit.

The above configuration is characterized in that the output of the optical signal transmitted from each optical transmission circuit corresponding to each center wavelength is controlled by the electric signal transmitted by each O / E converter. In particular, the intensity of the optical signal transmitted from each optical transmission circuit demultiplexed by the optical demultiplexer is made equal, and the optical signal intensity after being amplified by the optical fiber amplifier becomes substantially equal between each optical transmission circuit. The output of the optical signal is controlled as described above. Also, separately from the above, the intensity of the optical signal transmitted from each optical transmission circuit demultiplexed by the optical demultiplexer is multiplied by a transmission loss corresponding to each center wavelength of the optical transmission line measured in advance, The output of the optical signal transmitted from each optical transmission / reception circuit is controlled so that the intensity of the optical signal for each center wavelength after transmission through the optical transmission line is calculated so that the intensity of the optical signal becomes substantially constant. It is characterized by:

[0011]

According to the present invention, the above-mentioned problem, which is concerned when the wavelength division multiplexing transmission method is applied to an optical communication system using an optical fiber amplifier, is solved by using an optical splitter between the optical fiber amplifier and the optical fiber transmission line. A part of the light that has been arranged and branched is split into optical signals of the respective center wavelengths by an optical splitter.
The problem is solved by converting the signal into an electric signal by the / E converter and controlling the optical signal output of each optical transmission circuit.

That is, in the configuration of the present invention, the optical output of each optical transmission circuit is not controlled simply by amplifying the optical signal from the wavelength dependency of the optical fiber amplifier, but the optical amplification is performed by the optical fiber amplifier. The obtained amplified light is demultiplexed as it is by an optical demultiplexer, and each optical transmission circuit is controlled based on the demultiplexed light. Therefore, not only can the output ratio of the optical signal of each wavelength transmitted to the optical fiber transmission line be accurately detected, but also each of the optical signals of a plurality of wavelengths due to gain competition caused when the optical signal is input to the optical fiber amplifier at the same time. It is also less susceptible to fluctuations in wavelength gain and the like.

The output control to each optical transmission circuit can be controlled so that the output transmitted to the optical fiber transmission line is substantially equal, and the transmission loss of the optical fiber transmission line can be controlled in advance. It is also possible to measure the wavelength dependence and control it so that the intensities of the input optical signals to the optical receiving circuit become substantially equal in consideration of this. As a result, the intensities of the optical signals received by the respective optical receiving circuits can be made substantially equal, so that the isolation characteristics can be prevented from deteriorating.

[0014]

An embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical transceiver for wavelength division multiplexing transmission according to the present invention. An optical transmission circuit that converts an electric signal to be transmitted into an optical signal having each center wavelength and sends the signal, an optical multiplexer that multiplexes these optical signals, an optical fiber amplifier that amplifies the optical signal, an optical fiber transmission line, Each configuration of the optical demultiplexer and the optical receiving circuit is the same as the configuration of the conventional wavelength multiplex transmission optical transceiver shown in FIG.

Also in this embodiment, four wavelength division multiplexing transmission is used, and four optical transmission circuits 1a, 1b, 1c, which transmit optical signals having different center wavelengths from each other.
1d and an optical multiplexer 5 for multiplexing these four wavelength optical signals.
And an optical fiber amplifier 3 for amplifying the optical signal is disposed on the transmission side. On the receiving side, an optical fiber transmission line 4
A first optical demultiplexer 6 for demultiplexing the optical signal transmitted through the optical signal into an optical signal of each wavelength, and four optical receiving circuits 2a, 2b corresponding to each wavelength for converting the optical signal into an electric signal. 2c and 2d are arranged.

Furthermore, in addition to the above configuration, in the optical transmitter for wavelength division multiplexing transmission of the present invention, an optical branch for taking out a part of the amplified light is provided between the optical fiber amplifier 3 and the optical fiber transmission line 4. The vessel 7 is arranged. Here, a second optical demultiplexer 8 for demultiplexing the split optical signal into optical signals of respective wavelengths again, and four Os for converting the split optical signal into electric signals for each wavelength, respectively. / E converters 9a, 9b, 9c, 9d
Is located on the transmitting side. Each O / E converter is compared
Through the control circuit 10, the optical transmission circuit 1 of the corresponding wavelength
a, 1b, 1c, and 1d, respectively.

The O / E converters 9a, 9b, 9c and 9d convert the optical signals of each wavelength into electric signals, and
Input to 0. In the comparison / control circuit 10, the intensities of the optical signals after optical amplification obtained from the electric signals from the respective O / E converters are compared. Then, the intensity of the optical signal of each wavelength before amplification is set so that the intensity of the optical signal after the optical amplification becomes equal. In this setting, for example, an optical signal having a wavelength higher in intensity than an optical signal having another wavelength is set to have a lower intensity, and conversely, is set to have a higher intensity if the signal is low. After this setting, a control signal for controlling the optical output of each of the optical transmission circuits 1a, 1b, 1c and 1d is output from the comparison / control circuit 10 to the corresponding optical transmission circuit of each wavelength. Each optical transmission circuit 1a, 1b, 1c, 1d
The output of the optical signal to be transmitted is adjusted by the input control signal. For example, if the light source used in the optical transmission circuit is a semiconductor laser diode, the adjustment is performed by changing the injection current value. Whether or not the above-mentioned optical signal intensity is obtained from the set injection current value is monitored by receiving the backward output light from the semiconductor laser with a monitor photodetector or the like.

In such an optical transmission device, the value of the injection current to the optical semiconductor laser in the optical transmission circuit and the output value of the optical signal transmitted from the optical fiber amplifier 3 are measured in advance, or as described above. The output of the optical signal of each wavelength from the optical fiber amplifier 3 can be freely controlled by setting the injection current value while monitoring with a monitoring photodiode or the like. Even if the outputs of the optical signals from the respective optical transmission circuits are individually adjusted to be substantially equal, even if the optical fiber amplifier 3 has wavelength dependence of the gain and the output of the amplified light of each wavelength cannot be equalized, the present invention According to the configuration, the uniformity of the intensity of the optical signal can be easily realized. Therefore, if the output between the optical signals of each wavelength is adjusted to be substantially constant by the above method, the optical receiving circuits 2a, 2b, 2
The difference in the intensity of the optical signals received by c and 2d can be reduced, and the reception levels can be made substantially equal.

Further, even when the transmission loss of the optical fiber transmission line 4 has wavelength dependence, the transmission loss is measured in advance for each wavelength, and the output of the optical signal of each optical transmission circuit is considered in consideration of the loss. Can also be controlled. In this case, the intensity of the optical signal of each wavelength to the first optical demultiplexer on the receiving side can be made substantially equal. Needless to say, even if the output of the optical signal transmitted from each optical transmission circuit is controlled in consideration of the wavelength dependence of the insertion loss of the first optical demultiplexer 8 and other devices disposed on the transmission path. Good.

Next, the results of a transmission experiment performed using the optical transmitter for wavelength division multiplexing transmission of this embodiment will be described.

The center wavelengths λ ₁ , λ ₂ , λ ₃ , λ ₄ of the optical signal wavelength-division multiplexed in the optical transmitter of the present embodiment are
1.547 μm, 1.552 μm, 1.557 respectively
μm and 1.562 μm, and the excitation light source has a wavelength of 1.4.
8 μm light is used. A Mach-Zehnder type optical multiplexer using a quartz waveguide and an optical multiplexer are used for the optical multiplexer 15 for multiplexing the light having the center wavelength and the first optical demultiplexer 16 for demultiplexing. ing. The optical splitter 7 has a split ratio of 100: 1, and about 1% of the amplified light sent to the optical fiber transmission line is split into the second optical splitter. Like the first optical demultiplexer, the second optical demultiplexer 8 also uses a Mach-Zehnder type optical demultiplexer, and demultiplexes the optical signal of each wavelength.

In the above configuration, the optical fiber amplifier 3
, The intensity of the optical signal of each wavelength transmitted from, for example, +2.
It shall be set to 5 dBm. Here, the intensity of about 1% of the light split by the optical splitter 7 is measured by the O / E converter, and the intensity of the optical signal transmitted from the optical fiber amplifier 3 is calculated. In the initial state, each optical transmission circuit 1
The current values injected into the optical semiconductor lasers at a, 1b, 1c, and 1d and the gain of the optical fiber amplifier 3 are set, and the optical signal intensity is set. For example, assuming that the gain at the peak wavelength of the optical fiber amplifier 3 is set to 10 dB, if the injection current value is set so that the optical output from the optical semiconductor laser of the optical transmission circuit 1a is about −6 dBm. Good. However, here, the insertion loss of the optical multiplexer 5 is set to about 1.
It is calculated as 5 dB.

Thus, based on the gain of the optical fiber amplifier 3 set, each of the optical transmission circuits 1a, 1b, 1c,
The optical output of 1d is set, and in this state, all the optical transmission circuits of each wavelength are simultaneously driven, and wavelength division multiplex transmission is performed. As described in the description of the related art, the optical fiber amplifier 3 has wavelength dependence of gain, and when an optical signal having a plurality of different center wavelengths is input, gain competition occurs. The gain of the signal does not always match the set gain. For this reason, in the initial state, the intensity of the optical signal transmitted from the optical fiber amplifier 3 does not match at each wavelength. In the conventional configuration in which the optical signal intensity from the optical fiber amplifier is not fed back at all, if the wavelength is the above,
An intensity difference of about 4 dB occurs between the wavelength at which the optical signal intensity is the maximum and the wavelength at which the optical signal intensity is the minimum.

On the other hand, according to the optical transmitting apparatus of the present invention, the intensity difference can be reduced to 0.5 dB or less.
As a result, the isolation characteristics between the optical receiving circuits are also 20
dB or more can be secured, and problems such as deterioration of transmission quality due to the influence of optical signals of adjacent wavelengths can be prevented.

In the above embodiment, the optical output of each optical transmission circuit is adjusted so that the intensities of the optical signals transmitted from the optical fiber amplifier 3 are matched. In the case where a difference in transmission loss occurs between the wavelengths because the one optical demultiplexer 6 and other optical components are disposed in the transmission line, the control of the optical transmission circuit is performed in consideration of the loss difference. May be performed. For example, in the above embodiment, if the transmission loss of the optical fiber transmission line 4 for the light of the wavelength λ ₁ is 1 dB larger than the transmission loss of the other wavelengths, only the optical signal of the wavelength λ _{1 has} the light of the other wavelength. An optical signal may be output at an intensity 1 dB greater than the signal. Further, in this embodiment, the wavelength division multiplexing transmission of four wavelengths has been described. However, the present invention is not limited to four wavelengths but can be applied to three wavelengths or less or five wavelengths or more.

In the above configuration, when the wavelength interval is relatively wide, a grating type or an interference film filter type can be used for the optical multiplexer or the optical demultiplexer. Further, an O / E converter for converting an optical signal of each wavelength split by the second optical demultiplexer into an electric signal may be incorporated in each corresponding optical transmission circuit and integrated.

[0028]

As described above, the optical transmission apparatus for wavelength division multiplexing transmission of the present invention has an optical splitter for splitting a part of amplified light between an optical fiber amplifier and a transmission line. The split optical signal is split by a second optical splitter, converted into an electrical signal by an O / E converter arranged for each wavelength, and sent out from the optical transmitting circuit as a control signal. A configuration is used in which the output of an optical signal is individually controlled for each wavelength.

Thus, a desired optical signal output can be obtained regardless of the wavelength dependence of the gain of the optical fiber amplifier, and the reception level of the optical signal can be kept constant. As a result, the isolation characteristics between the optical receiving circuits can be significantly improved, and the occurrence of power penalty can be effectively prevented.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical transmission device for wavelength division multiplexing transmission of the present invention.

FIG. 2 is a configuration diagram showing a conventional optical transmission device for wavelength division multiplexing transmission.

[Explanation of symbols]

1a, 1b, 1c, 1d, 11a, 11b, 11c, 11d Optical transmitting circuit 2a, 2b, 2c, 2d, 12a, 12b, 12c, 12d Optical receiving circuit 3, 13 Optical fiber amplifier 4, 14 Optical fiber transmission Path 5, 15 Optical multiplexer 6, 16 First optical demultiplexer 7 Optical splitter 8 Second optical demultiplexer 9a, 9b, 9c, 9d O / E converter 10 Comparison / control circuit

Claims (4)

(57) [Claims] 1. A plurality of optical transmission circuits for converting an electric signal into a plurality of optical signals having different center wavelengths and transmitting the same, and an optical multiplexer for multiplexing the plurality of optical signals and transmitting multiplexed light. An optical fiber amplifier that amplifies the multiplexed light and sends out the amplified light to an optical transmission line; an optical bi-branch device that branches a part of the amplified light to extract a branched light; An optical demultiplexer for demultiplexing an optical signal having a wavelength, a plurality of O / E converters for converting the demultiplexed optical signal into a reconverted electrical signal corresponding to each wavelength; An optical transmission device for wavelength division multiplex transmission, comprising: a comparison / control circuit that compares signals and controls the intensity of each of the optical signals transmitted from the optical transmission circuit. 2. The intensity of the optical signal transmitted from the optical transmission circuit of each wavelength is controlled by the comparison / control circuit so that the intensity of the optical signal of each wavelength transmitted from the optical fiber amplifier becomes substantially equal. 2. The method according to claim 1, wherein
The optical transmission device for wavelength-division multiplex transmission as described in the above. 3. The intensity of an optical signal of each wavelength transmitted from the optical fiber amplifier and transmitted by the optical transmission line.
2. The optical transmission device for wavelength division multiplexing transmission according to claim 1 , wherein the intensity of the optical signal transmitted from the optical transmission circuit of each wavelength is controlled by the comparison / control circuit so that the values are substantially equal. 4. An optical fiber transmission line comprising: multiplying the intensity of an optical signal of each wavelength demultiplexed by the optical demultiplexer by a transmission loss corresponding to each center wavelength of the optical transmission line measured in advance; Calculating the intensity of the optical signal for each of the center wavelengths after transmission, and controlling the intensity of the optical signal transmitted from each of the optical transmission circuits so that the intensity of the optical signal becomes substantially constant. The optical transmission device for wavelength division multiplex transmission according to claim 1, wherein: